Zeolite structure

The zeolites are a subclass of molecular sieves. They are a class of crystalline aluminosilicates based on rigid anionic frameworks with cations trapped inside tunnels or cages.

Aswell as acting as molecular sieves, zeolites can also exchage their ions for those in a surrounding solution.

General formula for zeolite composition is:

Mx/n[(AlO2)x(SiO2)y].mH2O

Where cations M of valence n balance the charges on the aluminosilicate framework.

Zeolite type A is based on sodalite units joined by O bridges between the square faces. Eight linked together in this way have a large central cavity called an alpha-cage. These cages share octagonal faces with an open diameter of 4.2Å. Therefore small molecules such as water can fill them and diffuse through them. However, the cages are not able to allow entrance of molecules with van der waals diameters greater than 4.2Å. (fig. 21)

The charge on the framework in zeolite type-A is neutralised by the presence of Na+ ions and the formula is;

Na12[(SiO2)12(AlO2)12].27H2O

Numerous other cations can be introduced by ion exchange with aqueous solutions, which is a very important property of zeolites. For this reason they are commonly used in laundry detergent to remove ions which harm the effectiveness of the surfactan.

Solide State Chemistry, Oxford Univ
http://www.chem.ox.ac.uk/vrchemistry/solid/Page25.htm